1. Field of the Invention
The invention relates to a frequency-selection device having a first input intended to receive a radio-electric signal whose frequency is referred to as radio frequency, this radio-electric signal being selected from a number of signals having frequencies which are situated in a given range, said device having a second input intended to receive a control signal defining the frequency of the radio-electric signal to be selected, a first output intended to supply a signal whose frequency, referred to as intermediate frequency, is fixed, and a second output intended to supply a signal indicating that the radio frequency corresponds to the frequency defined by the control signal, said device comprising:
an oscillator having a tuning input intended to receive a tuning signal, and an output intended to supply a signal whose frequency depends on the value of the tuning signal, PA1 a mixer having a first input constituting the first input of the device and intended to receive the radio-electric signal, a second input connected to the output of the oscillator, and an output constituting the first output of the device and intended to supply a signal whose frequency is equal to the difference between the frequency of the signal received at the first input and that of the signal received at the second input, PA1 a frequency divider having a first input connected to the output of the oscillator, a second input constituting the second input of the device, and an output intended to supply a signal whose frequency is equal to the frequency of the signal received at the first input, divided by a number defined by the control signal received at the second input and, PA1 a phase detector having a first input intended to receive a reference signal having a fixed frequency, a second input connected to the output of the frequency divider, and an output connected to the input of the oscillator. PA1 a modulo-M counter having a counting/initialization authorization input connected to the output of the flip-flop, a clock input intended to receive the reference signal, and P outputs, with M=2.sup.P, an active level at the counting/initialization authorization input authorizing the counter to count, an inactive level setting each output of the counter at the inactive level and, PA1 a detection module having P inputs connected to the P outputs of the counter, and an output constituting the second output of the device, intended to supply a signal which is set at the active level when all the outputs of the counter are at the active level and is maintained at this level until the counter receives an inactive level at its counting/initialization authorization input. PA1 N elementary modules each having a data input intended to receive an input signal, and a data output intended to supply a signal having a frequency which is K times smaller than that of the signal received at its input, the output of an elementary module being connected to the input of the next elementary module, the input of the first elementary module constituting the first input of the frequency divider, the output of the N.sup.th elementary module constituting the output of the frequency divider, each module also having a pulse output intended to supply a signal which, during each period of the output signal of the frequency divider, is active during the first active state of the input signal, and inactive otherwise and, PA1 a decoder having an input constituting the second input of the frequency divider, and N outputs each defining the ratio between the frequency of the input signal of an elementary module and that of its output signal,
2. Description of the Related Art
Such devices are commonly used in receiver units incorporated in television sets or radio telephones. In these devices, the phase detector synchronizes the signals which it receives at its two inputs. If it is unable to synchronize said signals, it supplies a tuning signal to the oscillator so as to correct the frequency of its output signal. When the frequency of the output signal of the oscillator allows selection of the radio-electric signal whose frequency corresponds to the required radio frequency which is defined by the control signal, which situation is referred to as "lock-on", the output signal of the frequency divider is in phase with the reference signal, and the phase detector no longer supplies the tuning signal to the oscillator. In the majority of existing frequency-selection devices, this absence of the tuning signal is detected and indicates the lock-on.
The tuning signal is generated in accordance with certain requirements. It is most frequently intended to start charge pumps which allow modification of the bias of certain elements of the oscillator. The particular shape of the tuning signal which results from these requirements does not allow a precise lock-on detection.